One good option is to use
nasal saline irrigation regularly, especially before you use your
CPAP machine. Pick one that is vigorous or forceful, and not the gentle mist kind. Saline acts a a decongestant, and should help to open up your nose and sinus passageways.
I would also find better ways of dealing with your stress. One simple options is to learn deep-breathing exercises. You can get DVDs or videos, or even take yoga or tai chi classes. Regular exercise is also very important.